Titanium coating on large area substrates such as powders or flakes can have applications as pigments in auto paint, corrosion protection, cosmetics, architectural and decorative use, and as functional materials and catalysts. Processes to form titanium-based coatings include physical deposition (PD), chemical vapour deposition (CVD), and powder immersion reaction assisted coating (PIRAC).
PD often requires low pressure operation and involves use of metallic precursors. PD is based on evaporating a target and transporting the vapour onto the surface of the substrate. PD is generally slow and expensive, and can be difficult to implement for coating powdery substrates. Examples of PD technologies can be found in U.S. Pat. Nos. 6,241,858 and 6,676,741 describing processes for coating powder samples to produce metallic pigments.
CVD is a heterogeneous process, which involves reacting reducible precursors with a reactive gas on the surface of the substrate, leading to deposition of a thin coating. Most CVD processes for deposition of Ti-based films start from titanium tetrachloride and proceed to producing subchlorides, and then reacting or dissociating the subchlorides to form coatings. Conventional CVD processes/reactors are usually not amenable to coating powders. Examples of CVD based processes for deposition of Ti can be found in U.S. Pat. Nos. 4,803,127 and 6,169,031, both pertaining to reduction of TiCl4 to subchlorides and then dissociation of the resulting subchlorides over a single non-powdery substrate.
Variants of CVD systems include fluidised beds, which have been used for production of coatings based on metal carbides and metal nitrides for applications in hard coating and corrosion protection; U.S. Pat. Nos. 5,171,734, 5,227,195 and 5,855,678 (Sanjurjo et al.) disclose a fluidised bed process based on reacting gaseous TiCl4 with Ti, Cr, Zr, Nb, Mo, Hf, Ta, Mo, Si and Al in a fluidised bed at temperatures between 200° C. and 1000° C. to produce titanium subchlorides followed by further gas reduction at the substrate surface to produce coatings based on carbides, nitrides and oxides. Possible downsides to this approach include the difficult nature of gas phase reduction, the high cost of reducing metals such as Ti, Cr, Zr, Nb, Mo, Hf, Ta, Mo, and Si, and for Al reduction, the high temperature range used.
PIRAC has been used for coating ceramic substrates, where a substrate is immersed in a metallic powder and heated at temperatures above 800° C. to cause the substrate surface to react with the powder and form a metallic skin. For example, Si3N4 flakes are immersed in a titanium powder and heated at temperatures above 850° C. to form a coating of Ti5Si3 and titanium nitride. PIRAC is mostly limited to high temperature substrate materials; substrates such as borosilicate glass flakes and soda-glass which are unstable at temperatures above 700° C. and are unsuitable.
It is advantageous to develop a low-cost process for depositing Ti-based coating on large area substrates such as powders and flakes. Such a process would be particularly desirable if it was capable of producing a range of Ti-based coatings on common powdery substrates without the environmental and cost disadvantages of existing technologies.